781721 has 2 divisors, whose sum is σ = 781722. Its totient is φ = 781720.

The previous prime is 781681. The next prime is 781733. The reversal of 781721 is 127187.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 583696 + 198025 = 764^2 + 445^2 .

It is a cyclic number.

It is not a de Polignac number, because 781721 - 2¹⁰ = 780697 is a prime.

It is a super-2 number, since 2×781721² = 1222175443682, which contains 22 as substring.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (781741) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 390860 + 390861.

It is an arithmetic number, because the mean of its divisors is an integer number (390861).

2⁷⁸¹⁷²¹ is an apocalyptic number.

It is an amenable number.

781721 is a deficient number, since it is larger than the sum of its proper divisors (1).

781721 is an equidigital number, since it uses as much as digits as its factorization.

781721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 784, while the sum is 26.

The square root of 781721 is about 884.1498741729. The cubic root of 781721 is about 92.1192923220.

The spelling of 781721 in words is "seven hundred eighty-one thousand, seven hundred twenty-one".